In the prior art, a belt type continuously variable transmission is sometimes used as a vehicle transmission. The belt type continuously variable transmission has a primary pulley which receives an engine rotation, a secondary pulley which outputs the rotation to drive wheels, and a V-belt for transmitting the rotation of the primary pulley to the secondary pulley. By adjusting the pulley ratio which is the ratio of the contact radius of the V belt relative to the primary pulley and secondary pulley (i.e. effective radius of the primary pulley and secondary pulley), the speed ratio of input and output rotation speed can be adjusted.
Tokkai Hei 03-181659 published by the Japan Patent Office in 1991 discloses an example of a control device for a belt type continuously variable transmission. The control device can improve controllability and can also easily cope with abnormalities. Due to this control device, a required primary pressure is computed based on the input torque and pulley ratio in a steady state (state where a fixed pulley ratio is maintained). An electrical signal of a target primary pressure according to a required primary pressure is inputted into a primary pulley control valve to control the primary pressure, and the pulley ratio is maintained by oil pressure. In a transient state (state where the pulley ratio is changing), the primary pressure is increased by the control valve of the primary pulley to perform an upshift, and the primary pressure is reduced to perform a down shift. However, since the primary pressure is decreased during a downshift, the torque capacity (maximum torque which can be transmitted) of the primary pulley may fall too much, and the belt may slip.
Tokkai 2004-92669 published by the Japan Patent Office in 2004 discloses a control device of the prior art for coping with this problem. Due to this device, in the steady state, a steady oil pressure which attains the present pulley ratio of the primary pulley and secondary pulley and the torque capacity of the belt, is supplied. During a downshift, a transitional oil pressure for attaining a target shift speed is applied to the secondary pulley to change the pulley ratio. A differential thrust required to vary the pulley ratio, is controlled according to the shift speed. (The term “differential thrust” is defined as the difference between the thrust required to steadily maintain a certain pulley ratio and a real thrust.) Hence, the pulley ratio is changed while the torque capacity of the pulley is always maintained, so the belt is prevented from slipping during a down shift.